The narrower the current pulse width of discharge current, and the larger the peak value of the discharge current, the more the speed of electric discharge machining can be improved. Referring to FIGS. 7A and 7B, there will be described a prior art example of an electric discharge machining power source unit that can give a high peak current value with such a narrow pulse width.
The positive terminal of a DC power source 10 and a workpiece W are connected through a first switching element T11, while the negative terminal of the DC power source 10 and a machining electrode P are connected through a second switching element T12. Further, the negative terminal of the DC power source 10 and the workpiece W are connected through a reversely-connected first diode D11, while the positive terminal of the DC power source 10 and the machining electrode P are connected through a reversely-connected second diode D12.
The first and second switching elements T11 and T12 are each formed of a FET. As shown in FIG. 7B, their respective gates G11 and G12 are controlled by means of a first switching element driver circuit 11 and a second switching element driver circuit 12, respectively. The first and second switching element driver circuits 11 and 12 are driven by means of pulses with a given pulse width delivered from a pulse signal generator circuit 13 for current peak value setting.
When conditions for electric discharge between the workpiece W and the electrode P are fulfilled, the pulses with the set width are delivered from the pulse signal generator circuit 13 for current peak value setting. These pulses are applied to the respective FET gates G11 and G12 of the first and second switching elements T11 and T12 through the first and second switching element driver circuits 11 and 12, thereby turning on both these switching elements T11 and T12.
Thereupon, a discharge current J0 from the DC power source flows through the first switching element T11, workpiece W, machining electrode P, and second switching element T12 to the DC power source 10. When the pulses with the set width from the pulse signal generator circuit 13 for current peak value setting then die out, both the switching elements T11 and T12 are turned off.
After both the switching elements T11 and T12 are turned off, a current J1 (=J0) produced by induced energy that is accumulated by inductance in this electric discharge circuit is fed back to the DC power source 10 through the first diode D11, workpiece W, machining electrode P, and second diode D12.
The workpiece W is subjected to electric discharge machining using the current J0 (machining current) that flows between the workpiece W and the machining electrode P in this manner.
Referring now to FIGS. 8 and 9, there will be described discharge currents (machining currents) that are obtained when the power source unit shown in FIGS. 7A and 7B is operated in the aforesaid manner. FIG. 8 shows current waveforms for the case where the voltage of the DC power source is raised, and FIG. 9 for the case where the voltage of the DC power source 10 is lowered.
When both the first and second switching elements T11 and T12 are turned on by means of pulse signals (with a pulse width t1) from the pulse signal generator circuit 13 for current peak value setting (see (a) and (b) of FIG. 8 and (a) and (b) of FIG. 9), the voltage of the DC power source 10 is applied between the workpiece W and the machining electrode P. Thereupon, the current J0 that flows between the workpiece W and the machining electrode P is caused to increase with time by inductance in the circuit of FIG. 7A. If the voltage of the DC power source 10 is high, in this case, the current J0 rises sharply (see (c) of FIG. 8). If the voltage is low, on the other hand, the current J0 rises gently (see (c) of FIG. 9).
When both the first and second switching elements T11 and T12 are turned off, the feedback current J1 (FIG. 7A) flows through the first and second diodes D11 and D12. The falling speed of this current also increases if the voltage of the DC power source 10 is high ((d) of FIG. 8), and decreases if the voltage is low ((d) of FIG. 9).
As seen from the above description, a peak value Jp of the discharge current is determined by the output voltage of the DC power source 10 and the pulse width t1 of the pulse signals delivered from the pulse signal generator circuit 13 for current peak value setting. On the other hand, a fall time t2 of the feedback current J1 after concurrent deactivation of the first and second switching elements T11 and T12 is determined by the voltage of the DC power source 10. The sum of the times t1 and t2 is equal to a machining pulse width Pw.
FIG. 10 shows relations between the current peak value setting pulse width t1, current peak value Jp, and DC power source voltage V. FIG. 10 indicates that the lower the voltage V of the DC power source (V1&gt;V2&gt;V3), the wider the current peak value setting pulse width t1 must be made in order to obtain the same current peak value Jp.
FIG. 11 shows machining pulse widths Pw1 and Pw2 for the cases where V1 and V3 (V1&gt;V3) are used as the voltage V of the DC power source 10 to obtain the same current peak value JP. FIG. 11 indicates that if the voltage V of the DC power source is high (that is, where V=V1), both the current peak value setting pulse width t1 and the fall time t2 of the current are reduced (as compared to the case where V=V3 is given), so that the machining pulse width, which is equal to t1+t2, is reduced (i.e., Pw1&lt;Pw3).
In order to obtain the target machining pulse width (t1+t2) and the current peak value Jp as machining conditions, as described above, the value of the voltage V of the DC power source must be determined besides the current peak value setting pulse width t1.
There is a case in which the DC power source is divided between a DC power source (main DC power source) for applying the discharge current and a DC power source for energy regeneration, the voltage value V of the main DC power source being fixed to be constant, and the voltage value of the DC power source for energy regeneration being adjustable (i.e., the value of t2 being adjustable), in order to solve this problem. This case will now be described with reference to FIG. 12.
When both first and second switching elements T11 and T12 are turned on, a voltage V of a main DC power source 10 is applied between a workpiece W and an electrode P, and a discharge current J0 flows there. When both the first and second switching elements T11 and T12 are turned off, a current J1 produced by induced energy that is accumulated by inductance in the circuit of FIG. 12 returns to a power source 20 for energy regeneration through a first diode D11, the workpiece W, the machining electrode P, and a second diode D12.
Since the voltage of the main DC power source 10 in FIG. 12 is fixed to be constant, a current peak value Jp is determined by a pulse width with which both the first and second switching elements T11 and T12 are turned on, that is, a current peak value setting pulse width t1. On the other hand, a fall time t2 of the current is determined by the voltage of the DC power source 20 for regeneration. This time t2 can be adjusted by regulating the voltage of the DC power source 20 for regeneration. In consequence, a machining pulse width (t1+t2) can be selected by regulating the voltage of the DC power source 20 for regeneration.
Referring now to FIGS. 13 and 14, there will be described the way the machining pulse width (t1+t2) can be adjusted by means of the electric discharge machining power source circuit shown in FIG. 12.
FIG. 13 is a diagram showing current waveforms and the like for the case where the voltage of the DC power source 20 for regeneration is raised in the electric discharge machining power source circuit shown in FIG. 12. FIG. 14 is a diagram showing current waveforms and the like for the case where the voltage of the DC power source 20 for regeneration is lowered.
The current peak value Jp is determined by the current peak value setting pulse width t1 with which the switching elements T11 and T12 are turned on and the voltage (constant) of the main DC power source. Thus, if the current peak value setting pulse width t1 is common to both cases of FIGS. 13 and 14, therefore, the current peak value Jp is also a common value.
On the other hand, the fall time t2 of the current is determined by the voltage (variable) of the DC power source 20 for regeneration. Therefore, the current fall time t2 is shortened in the case of FIG. 13 (where the voltage of the DC power source 20 for regeneration is high) and is lengthened in the case of FIG. 14 (where the voltage of the DC power source 20 for regeneration is low). Accordingly, the machining pulse width Pw (=t1+t2) can be adjusted by regulating the voltage of the DC power source 20 for regeneration.
In any of the conventional methods described above, however, the discharge current waveforms are triangular, thus constituting a substantial hindrance to the improvement of the electric discharge machining characteristics.
Electric discharge in electric discharge machining starts with finding out an infinitesimal conduction path of scores of micrometers or less in a gap between the electrode and the workpiece, then supplying a pulse current there, and forcing the infinitesimal conduction path or infinitesimal portions of the electrode and the workpiece in contact with it to transpire or fuse and scatter by means of heat energy produced there. Thus, the degree of the transpiration or fusional scattering in the infinitesimal portions is determined by the level of the rate of change in the pulse current with respect to time, that is, the levels of the current having a sharp leading edge and the current peak value, the thermal properties of the electrode, workpiece material, etc., the cooling properties of an insulating solution, and the like.
If the workpiece is formed of a material with low electric resistance, heat generation attributable to Joule heat is low. If it is formed of a material with high thermal conductivity, heat generation or temperature rise in the infinitesimal portions can be restrained. Further, a material with substantial fusion latent heat and high fusion temperature cannot easily fuse if it is heated. A material that is highly viscous when it is fused is not ready to scatter if it is fused.
In actual machining, the combination of these conditions results in phenomena including low machining speed, high or low surface roughness, liability to short-circuiting, low machining efficiency, liability to concentrated electric discharge, etc. In wire electric discharge machining, the combined conditions result in frequent short-circuiting, frequent breaking of wire, etc.
Conventionally, the possibility of short-circuiting is eliminated by using an alloy material with low fusion temperature or low fusion latent heat or low fused-state viscosity for the electrode. This material, which may be an alloy of brass or the like, entails dissipation of the electrode and other problems, so that it is not often used in any other apparatuses than wire electric discharge machines and high-speed perforating machines. Wire electrodes for wire electric discharge machining include special wires that are coated with a material having low fusion temperature and low fused-state viscosity. They have an effect to prevent the aforesaid short-circuiting, thereby improving the machining efficiency.
After electric discharge is started, the insulating solution around the electric discharge is evaporated to form bubbles that expand suddenly. The fused portions are scooped out by the counteraction of the internal pressure. While the fused portions gradually spread as the electric discharge continues, the density of the generated internal pressure gradually lowers as the bubbles expand. Accordingly, there is a maximum value of scooped margin that depends on the material and discharge time, so that machining efficiency may lower when the discharge time (pulse width) is either short or long. Application for an unduly long discharge time (pulse width) is undesirable because the applied energy is consumed for heating and fusing of the electrode and the workpiece, in particular, so that the work surface tends to involve a thick fused layer.
It is desired, therefore, that each of various conditions including the discharge time (pulse width) as machinability, as well as the levels of the current peak value and the current rise speed as electric discharge starting capability, can be selected independently of one another, according to differences in an electrode, a workpiece, and a themal property of insulating solution. In regions with very narrow pulse widths, as mentioned before, efficient machining cannot be effected with use of pulses that have triangular waveforms X+.